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At boot time, a workstation that has been set to boot from PXE will issue a BOOTP request via the network. Once the request is received, the DHCP Server will supply an IP address to the machine, and the DNS server will point the client computer to the RIS server, which in turn will issue a disc boot image (often called the "OS Chooser").
Once all the boot and system drivers have been loaded, the kernel starts the session manager (smss.exe), which begins the login process. After the user has successfully logged into the machine, winlogon applies User and Computer Group Policy setting and runs startup programs declared in the Windows Registry and in "Startup" folders.
The boot loader; performs basic system initialization options such as loading the hardware abstraction layer and boot-time device drivers, prior to passing control to the Windows kernel. In versions prior to Vista, NTLDR and IA64ldr also display menus to the user if multiple operating systems are defined in boot.ini, or if F8 is pressed.
Some boot loaders can also load other boot loaders; for example, GRUB loads BOOTMGR instead of loading Windows directly. Usually, a default choice is preselected with a time delay during which a user can press a key to change the choice; after this delay, the default choice is automatically run so normal booting can occur without interaction.
The difference between the boot loader and second stage loader is the addition of checking code to trap paper tape read errors, a frequent occurrence with relatively low-cost, "part-time-duty" hardware, such as the Teletype Model 33 ASR. (Friden Flexowriters were far more reliable, but also comparatively costly.)
With boot times more of a concern now than in the 1980s, the 30- to 60-second memory test adds undesirable delay for a benefit of confidence that is not perceived to be worth that cost by most users. Most clone PC BIOSes allowed the user to skip the POST RAM check by pressing a key, and more modern machines often performed no RAM test at all ...
GRUB 2, elilo and systemd-boot serve as conventional, full-fledged standalone UEFI boot managers (a.k.a. bootloader managers) for Linux. Once loaded by a UEFI firmware, they can access and boot kernel images from all devices, partitions and file systems they support, without being limited to the EFI system partition.
On system with BIOS firmware, the BIOS invokes MBR boot code from a hard disk drive at startup. The MBR boot code and the VBR boot code are OS-specific. In Microsoft Windows, the MBR boot code tries to find an active partition (the MBR is only 512 bytes), then executes the VBR boot code of an active partition.